Conventionally, devices which each prevent collision by detecting obstacles present around a vehicle by using ultrasonic sensors (also referred to as sonars) mounted on the vehicle are known (see, for example, PTL 1).
The ultrasonic sensors can detect a distance to each obstacle based on a time from transmission to reception and a sonic speed by transmitting an ultrasonic wave and then receiving the returning ultrasonic wave reflected by the obstacle. By activating a plurality of ultrasonic sensors in one detection area and causing each ultrasonic sensor to measure a distance to the obstacle, it is possible to detect a position of each obstacle by trilateration.
When an obstacle is detected by using ultrasonic sensors, a curbstone which protrudes from a road surface is erroneously detected as the obstacle in some cases. A technique which controls detection areas of ultrasonic sensors to prevent this erroneous detection is known (see, for example, PTL 2).
According to PTL 2, when a reception gain of a distance sensor can be controlled and a vehicle posture changes, whether or not a ground is included in the detection area of the distance sensor is determined based on the posture of the vehicle. When the ground is in the detection area, the reception gain of the distance sensor is decreased and the detection area is reduced such that the ground is not included in the detection area. Further, even when the ground is not included in the detection area, if an obstacle is detected, the detection area is restricted such that a height corresponding to a curbstone is not included in the detection area. Consequently, the curbstone is not detected as the obstacle, so that reliability to detect the obstacle improves.